Commercial automotive coatings, that is, coatings for automobiles and trucks (motor vehicles), are typically so-called color-clear composite coatings comprising an opaque pigmented colored basecoat that is applied either to a cured electrodeposited primer layer or to a cured primer surfacer layer that has been previously applied over the electrodeposited primer layer. A transparent coating called a clearcoat is then applied to the basecoat. Such a composite coating has outstanding gloss and distinctness of image, that is, the ability to reflect an image in a sharp, well-defined manner.
For various reasons, the composite coating may have a defect that needs to be repaired before the motor vehicle leaves the factory. Typically, the repair is done on the assembly line shortly after the motor vehicle exits the assembly line.
Most of the basecoats that are used in automotive assembly contain an aminoplast curing agent and are deposited from an aqueous coating composition. These basecoats are typically used for the repair coating. The defect is removed from the damaged area, the basecoat applied, and a 2K clearcoat comprising a polymeric polyol, such as a (meth)acrylic polyol, and a polyisocyanate curing agent is applied to the uncured basecoat via a wet-on-wet process and the composite coating cured at about 140° C. 2K clearcoats are typically used in the repair coating even though they are not used as the original clearcoat in the painting process. This is because they are easy to apply, cure quickly and provide outstanding clearcoat properties in the damaged area. Although the 2K clearcoat composition can be cured at a temperature as low as 60° C., higher temperatures are used because the aminoplast curing agent in the basecoat does not cure well below 140° C.
For the basecoat that is used in the repair coating process, it would be desirable to have a coating composition that is aqueous and cures well at temperatures as low as 60° C., such that the 2K clearcoat can be applied wet-on-wet to the basecoat and the composite coating cured at temperatures as low as 60° C.